On Sat, Aug 26, 2006 at 10:29:24AM -0600, Jim Borynec wrote:
Hi All,
I’ve installed/compiled gnuradio on an Ubunto box. Together with a USRP
and a 3 foot length of wire,
I’ve managed to turn my computer into an FM radio.
I really appreciated all of the tutorials and how-to’s that people had
prepared for me.
It made things a lot easier.
Great, glad to hear that it’s working.
Next, I’d like to play with two antennas and compare the time latencies
between them.
(this would give me some primitive direction finding abilities.)
I can put the antenna’s out in the back yard, but I don’t want to move
my computer,
OK.
Are there any faq’s on “roll your own” FM antennas?
Is it simply a matter of taking a coax cable, cutting bare the center
wire for a metre and
plugging it into my basic RX daughter board?
I’d start with a couple of 1/4 wave monopoles.
1 wavelength at 100 MHz is about 3m.
1/4 of that is 75cm.
You’ll want to space the antennas 1/2 wavelength apart.
Mount your monopoles vertically over some kind of ground plane.
I used “hardware cloth” from Home Depot.
http://comsec.com/2x2-antenna.jpg
The daughterboard has a jack for the antenna, but it looks too small for
a normal cable TV sized coax cable. what type of connector do I use?
SMA. Jameco carries a variety of SMA-to-foo connectors.
http://www.jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?langId=-1&storeId=10001&catalogId=10001&categoryId=304550
If I keep the wires the same length then any time differencial in
signals must be due to the different speed of light latencies in
reception, correct?
Yes, assuming the group delays through all the analog h/w are the
same (reasonable assumption for Basic RX).
How far from the computer can I move the USRP and still get decent
throughput? can I put the antennas in the back yard and run them to
a USRP by the side of the house (where there is a plugin), then run
a USB wire to my computer inside the house?
I think USB 2.0 is spec’d to 2 meters.
You can run the coax from the antennas a pretty good way depending on
the type you use. There are tables that will give you the
loss/distance as f(freq). Coax | The Orchard City Amateur Radio Club
Is there an easier way to do this stuff? I saw a picture of 4
antenna on the gnuradio passive radar page. That is sort of the
configuration I’ll eventually want to work towards.
Two people built it in an afternoon out of pvc pipe, zip ties,
hardware cloth and a few odds and ends from Radio Shack.
Thanks in advance for any help.
You’re welcome! Have fun and let us know how it goes!
FYI, there’s code to gather data from 4 antennas
in gnuradio-examples/python/multi-antenna/*.py
If you want to try to DF in the 2-meter band, take a look at using a
Ramsey PR100 on each input. They should work fine in front of the
Basic Rx and will provide gain and selectivity that you’ll want.
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=PR100
You’ll need to calibrate the system to handle the differences between
the two inputs.
Eric